With the development of industrial and agricultural production, agricultural environmental pollution, especially soil heavy metal pollution problem has become more and more prominent. It is estimated that up to 12 million tons of grain is polluted by heavy metals in China every year, resulting in direct economic losses of more than 20 billion Yuan. The soil environmental quality is directly related to the safety of agricultural products; the quality and safety problems of agricultural products and mass incidents caused by soil pollution increase year by year and become an important factor affecting people's health and social stability. For most heavy metal pollution of farmland soil in China, it is difficult to use a single repair technology to solve the problem. It is urgent to develop a method to control the heavy metals in farmland soils which can be widely applied, low-cost, and can reduce the content of heavy metals in agricultural products.
Heavy metal passivation technology is considered to be one of the most effective measures to control the heavy metal pollution in farmland soil. By the application of a soil-friendly conditioning agent, passivation of heavy metals in pollution soil, and reducing the absorption of heavy metals in crops, to achieve both the control of the heavy metal pollution of agricultural soil and the production, is a new idea of soil heavy metal control, which is both economic and safe. Soil heavy metal passivation technology has a relatively low investment, a high repair efficiency, and a simple operation; it has better advantages for remediation of large areas of low-level heavy metal pollution, which can meet the current soil pollution control of agricultural soil and to guarantee the safety needs of agricultural products in China.
In recent years, biochar for the passivation of heavy metal pollution soil has gained more and more attention. Because the biochar contains a large number of carboxyl groups, hydroxyl groups and acid anhydride groups, as well as negative charges, a large specific surface area, a wide range of raw materials and a low price, biochar has gained great attention and extensively studied in soil heavy metal remediation. Biochar can increase the pH value of acidic soil and increase the cation exchange capacity, so as to adsorb pollutants and heavy metals in the soil and reduce the accumulation of Cd, Pb and Zn in the body of crops. However, since the mobility of As increases with the increase of the soil pH, it tends to bind to Fe, Al, and Mn oxides and hydroxides with anion exchange sites in the soil, which implies that the addition of biochar to the soil is not necessarily to control the bioavailability of As. As founded by Zheng et al., in researches, the migration ability of Cd, Zn, Pb and As in soil is significantly affected by the application of biochar to pollution rice paddy soil; the concentrations of Cd, Zn and Pb in rice roots are reduced by 98%, 83% and 72%, while the concentration of arsenic is increased by 327%. Because most of the farmland soil is of heavy metal combined pollution, this also poses a challenge to the application of biochar in heavy metal passivated farmland.
Although in our previous patent, “an iron-based biochar material, the preparation process therefor and use thereof in soil pollution control (Application number: 201410538633.8)”, the iron-based modification of the biochar can be a good solution for the simultaneous passivation of arsenic-cadmium combined pollution in soil; but its preparation process is relatively complicated. At the same time, because most of the biochar materials have a loose porous structure, and will soon react with the soil solution, once applied in soil, and the adsorption and fixation of heavy metals will reach saturation in a very short period of time. Because of large buffer performance of soil, the heavy metal ions in the soil solution will be released from soil colloids or soil minerals slowly after having been adsorbed by biochar. Therefore, if the biochar powder is directly applied to the farmland, a larger amount of the biochar is often used, in an amount of 1 ton or even several tons per acre, so as to have a better inhibition of heavy metal absorption and accumulation effect in the current season rice. To the next season, when the rice is planted, the same high dose of biochar is also needed, which will not only result in waste of resources, but also increase labour input. Moreover, biochar is relatively light, so the direct application tends to results in dust, which not only will cause waste, but will also have adverse effects on the health of users; furthermore, the application in the paddy field will also tends to result in floating on the water, and to accumulation at the downwind area due to the uneven application, and also the biochar may be flowed away by drainage water, thus leading to waste.